Eastern equine encephalomyelitis virus (EEEV) is the most pathogenic arbovirus endemic to the USA. Human cases of EEE, although sporadic and difficult to predict, are very serious, with over 35% of symptomatic cases being fatal and most survivors suffering long term neurological complications. Florida has the most EEEV activity in the US, with 25% of all reported human cases, and recent evidence suggests that Florida is a reservoir from which EEEV is disseminated to other states. There is no effective treatment or FDA approved vaccine for EEEV. Thus, prevention of infection relies upon surveillance to detect viral transmission, followed by measures to try to prevent the spread of the virus to humans. The effectiveness of these measures rests upon an understanding of the dynamics of EEEV transmission, and this understanding remains limited. In particular, researchers have been unable to determine how and where EEEV over-winters, and how it transitions from its winter cycle into enzootic amplification in avian hosts. Gaining an understanding of the mechanisms of over-wintering and early-season amplification of EEEV in Florida will allow us to predict when and where these processes may be occurring. This will lead to the possibility of developing strategies to interrupt transmission early in the season, which may lessen or prevent outbreaks of EEEV later in the summer, when mosquito densities are greater and the costs for intervention correspondingly higher. Furthermore, if Florida serves as the reservoir for EEEV for the rest of the country, early season interventions may protect much of the country by preventing dissemination of EEEV from Florida to other states. To accomplish this overall goal, the following Specific Aims are proposed: 1. To test the hypothesis that snakes are important over-wintering hosts for EEEV. 2. To delineate the hosts and vectors of winter and early spring enzootic transmission of EEEV in Florida. 3. To test the hypothesis that wading bird rookeries represent important foci for EEEV amplification. 4. To develop a risk map for early season EEEV transmission in Florida. Such a risk map will be invaluable in predicting foci of EEEV over-wintering and early season amplification, permitting the development of effective early season intervention strategies.